A wireless communications network (WCN) manages mobility of a wireless mobile device by collecting radio information about the network. From the advent of location-based services, this radio information has been used to provide low and medium accuracy location estimates.
In non-softhandoff systems, the location of every active mobile in the network is known to the nearest serving cell and sector. The identification of the serving cell and serving sector can be converted to a location estimate by simple translation to a pre-established latitude and longitude for the serving cell and/or sector.
Inclusion of the time or power-based range from the serving cell used by the WCN to maintain a radio link to the serving cell provides a method for refining the basic serving cell location with minimal calculation.
A further refinement of the cell/sector+ranging method using the mobile-collected network information is generally known as Enhanced Cell-ID (ECID). The ECID technique relies on the mobile unit's ability to record the power levels from the beacons (also known as pilots) of multiple potential handover candidate/neighbor cells. This technique adds a power-difference-of-arrival (PDOA) measurement to improve the serving cell ranging location estimate.
Since typically the signal power between the serving cell and the active mobile device is known by the WCN, the PDOA for ECID value is based on the received signal levels collected by the mobile for the serving cell's and at least three neighboring cell's beacons. Since the PDOA data collection requires visibility to two or more neighbor cell sites, location yield will be less than 100%. The effects of RF multipath, mobile receiver quality, and granularity of the measurement all act to reduce location accuracy for ECID.
ECID in GSM and LTE
In GSM, ECID is also known as Network Measurement Report (NMR) location. The NMR is generated by the mobile to provide the WCN with information regarding the serving and neighboring cells to facilitate handover as described in GSM/3GPP Technical Standard 05.08, “Radio subsystem link control” section 3 (Handover).
The Enhanced Cell ID positioning technique is standardized as “Timing Advance” positioning in 3GPP TS 43.059, “Functional stage 2 description of Location Services (LCS) in GERAN” section, section 4.2.1. In LTE networks the “enhanced cell ID method” is described in 3GPP TS 36.305, “Stage 2 functional specification of User Equipment (UE) positioning in E-UTRAN” Section 4.3.3.
In the example GSM system, the NMR contains the mobile generated Measurement Results. The purpose of the Measurement Results information element is to provide the results of the measurements made by the mobile station regarding the serving cell and the neighbor cells. The Measurement Results information element is coded as shown in GSM/3GPP Technical Specification 04.08, “Mobile radio interface layer 3 specification” section 10.5.2.20 (Measurement Report).
The mobile location center (MLC) uses NMR delivered cell-id (in GSM the Cell-Global-Identity (CGI) gives the cell and sector) as the geographical starting point. The timing advance (TA) value allows computation of a range from the starting point. The Received Signal Strength Indicator (RSSI) for the serving CGI is normalized with the current mobile power settings. The Reception Level (RxLEV) values for up to six neighboring CGI Broadcast Control Channel (BCCH) beacons are then normalized against the entered value of broadcast power. Using the CGI position, the TA-derived range, and the PDOA from three or more sites, a location estimate can be calculated.
Since ECID uses PDOA multi-lateration, the geographic layout of the neighbor cells also affects the quality of the location through geographic dilution of precision (GDOP). The limitation of only six neighbor cell RxLevel measurements present in the NMR limits accuracy by limiting potential GDOP reduction though receiver site selection.
Since the PDOA measurement requires averaging over multiple samples (the GSM NMR is transmitted by the mobile station periodically during an active call), latency is much higher than for other cell-ID based techniques.
Since the RSSI measurement is based on the variable power settings for the BTS, normalization of the RSSI before inclusion into the PDOA calculation requires knowledge of the BTS forward (downlink) power control settings from the GSM WCN.
Calibration may be used improve accuracy in ECID location systems. ECID Calibration can include the use of predictive RF propagation mapping and extensive drive testing to create a grid of CGI/RxLev “fingerprints”. By mapping the neighbor list and received signal levels over the coverage area, it is possible to achieve medium accuracy results within the range of 200-500 meters in networks having relatively high BTS density.
In U.S. Pat. No. 7,486,233, a single site ECID location system is taught where the power measurements from a single 3-sector Base Transceiver Station (BTS) with a serving sector and two co-sited sectors allow the formation of a sector limited timing range band and a directional angle from the BTS cell site.
The inventive techniques and concepts described herein apply to time and frequency division multiplexed (TDMA/FDMA) radio communications systems including the widely used IS-136 (TDMA), GSM, and Orthogonal Frequency Division Multiplexed (OFDM) wireless systems such as LTE, LTE-Advanced and IEEE 802.16 (WiMAN/WiMAX). The Global System for Mobile Communications (GSM) model discussed is an exemplary but not exclusive environment in which the present invention may be used.